This invention relates to an apparatus for controlling the temperature of the mobile phase in a fluid chromatographic system and, more particularly, to an apparatus wherein such temperature control is effected as the mobile phase enters and exits a liquid chromatographic column.
It is known that the separation process in the column of a liquid chromatograph is temperature dependent. That is, the chromatographic retention times of sample substances to be analyzed are a function of temperature. It is also known that the separation of the various substances in a chromatographic sample, known as the chromatographic resolution power, is also a function of temperature. Thus, temperature selection can be used in a systematic manner to adjust and optimize conditions for a specific separation problem. For example, there are known chromatographs which comprise means to adapt the temperature of an incoming solvent stream to that of the column to minimize chromatographically disturbing temperature gradients in the column bed. In many chromatographic applications, a temperature in the range between 25.degree. C. and 100.degree. C. is used. One device for controlling the temperature of the chromatographic mobile phase is disclosed by European Application EP-Al 0 043 946. In this apparatus, the separation column is arranged in a housing of heat insulating material in which air is circulated with a fan. The air circulating in the housing is heated as it flows past an electrical heating element; thereafter, the air flows past the column and brings it to an elevated temperature. In order to achieve pre-heating of the liquid flowing into the column, a portion of an ingoing capillary connected to the column entrance is arranged inside the heating element. The solvent stream leaving the column with the separated sample substances is then conducted to a suitable detector, for example to a photometric absorbance detector.
Many detectors used in liquid chromatography are temperature sensitive. The accuracy of these detectors can be significantly impaired by temperature effects. One common problem involves refractive index variations caused by differences between the temperatures of the solvent and the temperature of the detector. These variations lead to noise in the measured absorbance signal. Unfortunately, the level of this noise gets higher as the temperature difference between the solvent and the detector increases.
A further concern in liquid chromatography is external band spreading, which impairs chromatographic selectivity and resolution. External band spreading can be minimized by making the connection between the column outlet and the detector as short as possible. By maintaining a relatively short path between the column outlet and the detector, however, the solvent exiting a column operated at elevated temperature typically is hot enough upon reaching the detector to produce the above mentioned noise. On common way to cope with this problem is to provide the detector with extra heat exchangers which reduce the temperature difference by dissipating at least a portion of the heat into the air surrounding the detector. This approach, however, requires extra capillary tubes, which results in higher external bandspreading and makes the instrument more complex and costly.
Accordingly, there exists need for an apparatus for controlling the temperature of a liquid chromatographic mobile phase which has a less complex design and which at least reduces the problems associated with heat transfer.